Calculating Water Use, Direct and Indirect

Your household water meter only tells part of the story — what was directly used for washing, cooking and other tasks. But what about the water that was used to grow the food you ate for dinner? Or to manufacture the book you bought or the gasoline your car burned?

Researchers at Carnegie Mellon University have estimated this kind of direct and indirect water use — not for households, but for American industries. Their goal was to create a tool for better assessing the impact on water use of decisions made up and down the industrial supply chain, just as one might assess cost or carbon footprint.

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Chris Gash

Chris Hendrickson, a professor of civil and environmental engineering; Michael Blackhurst, a graduate student; and Jordi Sels i Vidal, a visiting researcher, used water data from the United States Geological Survey (from 2000) and applied an economic model to estimate direct and indirect use in paint manufacturing, fiber and yarn making, grain farming and about 420 other industrial sectors, big and small. Their findings are reported in Environmental Science and Technology.

Most of the direct water use, they report, is accounted for by the broad categories of power generation and agriculture. As for individual sectors, almost all use more water indirectly than directly. For example, in cane sugar refining, relatively little water is used at the mills. More than 95 percent is used indirectly, to grow the cane, generate electricity for the mills and in other parts of the supply chain.

“If you’re doing a lifecycle assessment, you want to make sure you include upstream activities that are water intensive,” Dr. Hendrickson said. “This tool should allow people to understand the implications of water use.”

A version of this article appears in print on April 20, 2010, on page D3 of the New York edition with the headline: Calculating Water Use, Direct and Indirect. Order Reprints|Today's Paper|Subscribe